1. Field of the Invention
The present invention generally relates to technology for modulating light to be transmitted and, in particular, it relates to an optical modulator employing coherent optical modulation and the manufacturing method thereof.
2. Description of the Related Art
An optical modulator is a device for generating the required optical power, color and the like in the passing light by changing optical parameters such as the transmission factor, refractive index, reflection factor, degree of deflection and coherency of light in the optical system according to the modulating signal. For example, with an optical modulator employing the interference of light, light waves having different optical paths are superposed with a movable reflection film in which the position thereof changes minimally pursuant to a modulating signal so as to change the intensity and color of light with the interference of both waves. With this kind of coherent optical modulator, a mechanism for accurately displacing the position of the movable reflection film becomes necessary.
For instance, with the optical modulator described in Japanese Patent Laid-Open Publication No. 2002-174721 (JP '721), an optical gap is formed by providing a sacrifice layer between the substrate (fixed reflection film) and movable reflection film, and thereafter removing this sacrifice layer via underetching.
Further, with the optical modulator described in the specification of U.S. Pat. No. 6,341,039, an optical gap is formed by using a substrate in which one face of a silicon wafer is oxidized and a silicon layer is further affixed to the face to which the oxidized layer has been formed, and performing underetching with this oxidized layer as the sacrifice layer.
Nevertheless, with the method described in JP '721, since the initial spacing of the optical gap will be prescribed based on the thickness of the sacrifice layer, there are cases where this initial spacing will show variations depending on the forming conditions of such sacrifice layer. Thus, fluctuations will also arise in the electrostatic force working between the drive electrode (fixed electrode) formed on the side of the fixed reflection film, and the movable reflection film, and it will become difficult to stabilize the drive. Further, with this method, the working process will become complicated since the processes of deposition and sacrifice layer etching are repeated.
Moreover, with the optical modulators described in the above prior art, they both form the optical gap by removing the sacrifice layer via underetching. In order to perform underetching to the sacrifice layer and to release the moving part forming the movable reflection film, it is necessary to form a release hole in the moving part for introducing the etching solution into the sacrifice layer. Thus, the area to which the electrostatic force works to drive the moving part will decrease, and the drive voltage will increase. In addition, when the optical gap is reduced, there are cases where sticking phenomenon will occur due to the surface tension of the etching solution between the movable reflection film and the fixed reflection film upon releasing the moving part.
Therefore, in order to overcome the foregoing drawbacks, a constitution that does not require the underetching of the sacrifice layer is being sought.